1. Field of the Invention
The present invention relates to devices for measuring motion and more particularly to devices for measuring strong motions in solid mediums such as soil, natural rock or concrete.
2. Brief Description of the Prior Art
The current invention offers improvements over previous methods to measure long-duration (over several milliseconds) medium motion in strong shock environments (that is, over 1,000 psi normal stress, over one foot of displacement, over 1,000 g's). There are two common methods for measuring these environments: with commercial accelerometers rigidly mounted to inside surfaces of protective canisters, or with commercial accelerometers mounted through shock-isolation systems to the internal surfaces of protective canisters. Examples of the rigidly-mounted systems are: Biaxial Accelerometer Mini-Canister and Biaxial Accelerometer Micro-Canister, both designed at the U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. (about 1977) by Andres Peekna; and the Air Force Weapons Laboratory's Micro-Epoxy Canister described in M. L. Winiarz, D. W. Raymond, and R. Paul, "Survey and Sourcebook of Test Instrumentation for the DNA DUGHEST Program," Technical Report No. DNA-TR-89-283, Defense Nuclear Agency, Washington, D.C., March, 1991. Examples of measuring systems which use shock-isolation mounts are described as follows: in Petersen, C. F. and Groethe, M. A., "Research on HML/ASH Environment--Task 6--Instrumentation Development, ASH," Technical Report No. DNA-TR-88-69, Defense Nuclear Agency, Washington, D.C., Feb. 26, 1988; Welch, C. R., "Silo Test Program II Instrumentation Evaluation Test," prepared for the Defense Nuclear Agency under "Targeting/Vulnerability of Structures," Subtask Y99QAXSC062, Work Unit 50, May, 1981; Welch, C. R., (Editor and Co-author), "Silo Test Program (STP) 3.5A Event, Volume 1--The Test Environment," WES TR SL-84-11. October, 1986; Welch, C. R. and White H. G., "Shock-Isolated Accelerometer Systems for Measuring Velocities in High-G Environments, Shock and Vibration," Bulletin 57, U.S. Army Engineer Waterways Experiment Station, Vicksburg, Miss. January, 1987; and White, H. G., "Development of a High-Range Particle-Velocity Gage," WES Technical Report SL-89-3, U.S. Army Engineer Waterways Experimental Station, Vicksburg, Miss. January, 1989.
The rigidly-mounted systems suffer from several limitations. First, since the internal accelerometers are rigidly attached to inner surfaces of the protective canisters, the accelerometers undergo straining of their bases as the protective canisters are strained by the normal stresses in the medium induced by the load source (e.g., an explosion). This strain causes a change in the electrical resistive characteristics of the accelerometers, which, in turn, causes a change in the ambient or baseline voltage of the resultant signals. This change degrades the usefulness of the measurements.
Second, the external dimensions, in the measuring direction of the instrument, of the protective canisters for the rigidly-mounted systems are relatively large (two inches or more). This causes these systems to have only relatively low-frequency, rigid-body response to the motions of the medium. See C. R. Welch, A Full-Engulfment Engineering Model, and Its Experimental and Numerical Verification, for the Response of a Rigid Body to Ground-Shock (Virginia Polytechnic Institute, 1993). This response, in-turn, causes these measurements to have decreased fidelity in defining the medium's motions. Third, the cable protection systems for the rigidly-mounted systems lack the robustness of the current invention. Fourth, the protective canisters of the rigidly-mounted systems are less robust than those of the current invention.
The shock-isolated systems also have several limitations. First, their external dimensions are as large or larger than the rigidly-mounted systems, and hence suffer from the second limitation cited for the rigidly-mounted system. Second, the shock-isolated systems degrade either the frequency response, or the amplitude response, of the internal accelerometers to the rigid-body motions of the protective canisters. This factor further reduces the fidelity of the measurements with regard to defining the motion of the medium. Third, the cable protection systems of the shock-isolated systems are inferior to the gauge of the current invention either in robustness or in flexibility.